It doesn't properly keep up with Android updates (both Fairphone OS and /e/OS) and lacks a good secure element and other hardware security features

/e/OS degrades security compared to AOSP, bundles privileged MicroG which makes connection to Google by default etc.

Linux/Phosh is based on a traditional Linux desktop OS which lacks system wide mandatory access control and an obligatory enforced app sandbox with a fine grained runtime permission model. Lots of its user space is using tech stacks written in memory unsafe code with much less exploit mitigations than modern Android.

It's not the same category of hardware and software.